since you had a 8600 I assume you have covered all these but just to be safe...
is your monitor on the right input? HDMI/DVI, are you plugged into the card and not the board?
have you tried the onboard video? is the onboard video disabled?
if you are using a different input then your 8600 have you tried a different monitor to see if that input is deffective?
I wouldn't think this would matter but a BIOS update might not hurt.
Well, my monitor is a Acer LCD and is a VGA to VGA cable, but I have an adapter I use. That doesn't seem to be the problem, because the card I have in my pc as we speak is using the adapter also.
I haven't ever tried the onboard video (haven't needed to, always had a card in that worked).
Like I said, Gateway's website said there isnt a BIOS update available for my computer.